When I sync ebooks to my ereader, they're set to be converted by Calibre into ePub. How do I disable this for some of the files, so that when I select 'Send to Device', some get transferred as ePubs and others remain as native PDFs?

When I sync ebooks to my ereader, they're set to be converted by Calibre into ePub. How do I disable this for some of the files, so that when I select 'Send to Device', some get transferred as ePubs and others remain as native PDFs?